In order to add textures, e.g. a metallic, a pearl, a marble texture or so on, on a surface of a synthetic resin product, it is known a method in which resin material is mixed with a variety of pigments for expressing the textures previous to be injection molded. The method has advantages that compared to a method of adding spray coating or painting process to a conventional non-textural surface of a synthetic resin product, a manufacturing cost is much lower and there is no environmental trouble in relation to the coating or painting process.
Meanwhile, in the method of mixing molten resin with the pigments previous to injection mold, it is important that the pigments should be distributed evenly in a surface of a product after the injection molding. Particularly, flake type pigments should be oriented in parallel to a surface of a product. As such, only if the pigments are evenly distributed and properly oriented, the pigments contained in resin reflect evenly much amount of light and thus a product with brilliant feel and high quality can be obtained.
However, at a region R (i.e. a weldline) in FIG. 1 where at least two flows of the molten resin meet within a mold in an injection molding, a rear vicinity of a core (reference symbol 14 in FIG. 2) for forming a hollow in a product or so on, pigments within molten resin turn back from a core layer (reference symbol C in FIG. 3) to a skin layer (reference symbol S in FIG. 3) due to the fountain flow effect.
By this phenomenon, a low content area (reference symbol A in FIG. 3) in which a content of pigment is relatively low than other areas is generated at a vicinity of the weldline. Particularly in a case that the pigment is not a polyhedron type but a flake type, an orientation of the pigment (reference symbol 21 in FIGS. 3 and 6) is perpendicular to or slant to a surface of a product and thus an amount of reflected light is markedly decreased and the portion is looked like a dark line, which results in a defective product.
FIGS. 1 and 2 show schematically an injection molding of a product by a conventional injection mold. Although a structure of an injection mold varies as a shape of a product to be manufactured, a mold with the simplest structure is described as an example in order to help understanding.
Referring to FIG. 1, a core plate 15 and a cavity plate 13 are combined to form a single mold 11 and a cavity 17 which reflects a shape of a product to be manufactured is formed in an inside thereof. In addition, a gate 13a for injecting molten resin 19 is located at both side portions of the cavity plate 13. A location of the gate 13a is determined to a most appropriate position in view of various factors, such as flow property of the molten resin 19, flow length or so on, when designing the mold 11. In addition, a number of the gate 13a may also be varied in some cases.
However, two flows of the molten resin come to meet in a case that the mold has multiple gates even if the mold is very simple or by a structure of a product having a hole, difference in thickness or so on even in a case that the mold has a single gate, and there are many cases that two flows of the molten resin 19 which are generated by being injected through each of the gates or by a structure of a product meet frequently each other in the cavity 17. In example, as shown in FIG. 1, the molten resin 19 injected through the gate 13 a at both side meet at a center portion while moving to fill the cavity 17 in the z-direction.
The position at which the molten resin having different flows meet as described above is referred as a weldline R. Since unlike the other portions a flow of the resin is not smooth at a vicinity of the weldline, various phenomena which are not desirable are occurred which is likely to result in generation of a defect in product.
The problem resulted from the weldline is much significant in a case that the molten resin is mixed with pigments for expressing a texture to a product.
In addition, it is highly possible that the weldline is generated at a rear side of a core (reference symbol 14 in FIG. 2) in a case that the core 14 is provided in the cavity 17 of the mold 11 even if the resin is injected through the single gate 13a unlike the case shown in FIG. 1.
FIG. 2 shows the mold 11 provided with the core 14.
Referring to FIG. 2, it will be seen that the molten resin 19 injected into the cavity 17 through the single gate 13a flows viscously in a direction of an arrow P to fill gradually the cavity 17. The molten resin 19 is divided by the core 14 and then meets again at the rear side of the core 14. In other words, flow divided by the core 14 come to join again at the rear side of the core 14.
Such joining results in the same problem as that occurred when the two flows (which are flowed in through different gates) meet as described through FIG. 1. In other words, it results in the weldline R.
FIGS. 3, 4 and 5 are for explaining the problem of the conventional injection mold shown in FIG. 1.
FIG. 3 shows flow property of the pigments 21 at the vicinity of the weldline R in FIG. 1. The molten resin is primarily a viscous fluid, and when it flows inside a flow path a center portion thereof flows quickest and a portion adjacent to a wall flows slower as it approaches to the wall. Generally, a flow layer at a center portion is referred as a core layer C and a flow layer at an outer portion (i.e. between the core layer and wall) is referred as a skin layer S.
Anyway, referring to FIG. 3, the core layers C which meet together while flowing in z-direction in the cavity 17 of the mold collide with each other and then flow back in the opposite direction (while spreading out toward a circumference as a fountain). At this time, the cavity is in a state of being filled with the molten resin and thus there is no inflow of the molten resin. Therefore, the molten resin, which flows back in the opposite direction after colliding and then is located in the skin layer, is not able to move any more and then solidified at the place to become a product.
Meanwhile, the pigments 21 mixed in the molten resin move along the flow of the molten resin and then stop after being oriented in a pattern shown FIG. 3. Thus, a low content area A in which a content of the pigment is very low is generated at upper and lower portions of the weldline. The low content area A is a portion in which the content of the pigments 21 is very low than the other portion, and looks as dark as a light reflecting area is decreased and causes, particularly in a case that the pigment 21 is a flake type as shown, a result that the texture is not expressed at such portion of a product.
FIG. 4 is a sectional view showing a state that the molten resin 19 having the flows is solidified inside the mold. As shown, it will be seen that the low content area A is located at a vicinity of the weldline R. When opening the cavity plate 13 from the core plate 15 and taking an injection molded product B, it will be easily seen that a center portion of the product B, i.e. the low content area A at the vicinity of the weldline R looks darker than the other areas as shown in FIG. 5. Such product is classified as a defective.
FIG. 6 is a sectional view taken along line Q-Q in FIG. 2 for explaining the problem of the conventional injection mold shown in FIG. 2.
Referring to the drawing, it will be seen that the low content area A is located at a side portion of the mold 14. The low content area A is generated by the same cause as the description in relation to FIG. 3 and is expressed as the weldline R on a surface of the product.